This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In Stage I, to evaluate the short term safety and tolerability of vicriviroc in combination with optimized background therapy which includes a ritonavir-boosted protease inhibitor regimen in HIV-infected, treatment experienced children and adolescents. In Stage I, to determine the multi-dose pharmacokinetics of vicriviroc in combination with optimized background therapy which includes a ritonavir-boosted protease inhibitor, and to determine the dose of vicriviroc that achieves a target Cmin in children and adolescents. In Stage II, to evaluate the long term safety and tolerability of the optimized dosage of vicriviroc in combination with optimized background therapy which includes a ritonavir-boosted protease inhibitor in HIV-infected, treatment experienced children and adolescents.